I have been playing my GK for about a year now, up until today under the visage that they were a army that you would charge up the field, while shooting your way there, and get into close combat ASAP. The problem was while playing under this assumption, I usually managed to wipe out half of my opponents army, but almost always ended up getting tabled. I was really close to just rage quiting the whole army when I came upon an article you wrote regarding Purifiers in Close Combat. You made a lot of good points about why they play much better as a mid-field defense army and I am working on a list to try them out in such a way now. The main problem I see with this is though is that it appears it would leave you absolutely helpless against a long range shooting army like Tau or Imperial Guard, as you may just end up getting riddled down by their fire if you stay put, and you will certainly end up dead with all your tanks popped if you try and do a slow moving advance.
This is a particularly big problem for me as I didn't know I was suppose to be playing grey-knights as a mid-field defense army until yesterday, and have no idea how to play that kind of army at all(though it does appeal to me in a way). So what is the fix for this? Do you deep strike some paladins/termies right up in their face? Do you put a bunch of deathcult+a few crusaders in a suicide chimera to get them up the field as fast as possible?
This is a question relevant to 6th edition as well, hence posting now. The main trick here against shooting armies is to get into the middle and then further up the board as quickly as possible. If we look at 24" threat ranges from midfield, they can hit pretty much anywhere. Do this you really need to ensure your mech survives for as long as possible tomake sure you can get to that midfield position - if going 2nd the best way is to line all your vehicles up where they get the most amount of cover from your opponent and then advance. The line ensures none of your movement is hampered by an immobilised or stunned vehicle and every vehicle gets the maximum distance. After that or if going first, you need to train your vehicles to ensure they are all getting cover from each other (explained here).
You lose a turn or two of shooting and will lose part of your army but gain some decent board control. Also bringing long-ranged shooting like Psyfledreads is going to help damage your opponent as you work your way forwards.
You don't need the deep-striking units to delay your opponent - that generally just hands them a unit which they can use all their short-ranged back up firepower (i.e. Kroot rifles, lasguns, etc.) to down and keep the heavy stuff hitting you as you move up. Rather, get your whole army up there ASAP - particularly against such armies where you will destroy them in CC regardless. Seems counter-intutitive compared to how Grey Knights normally play but CC is often the best answer for shooting armies like Tau or Guard. Just make sure you're not running across the board and leeroying into combat - use the above movement concepts and firing whilst you're moving in to weaken them and finish them off in combat.